Ok I'm at a crucial deciding point and I would be grateful for suggestions. Having taken all suggestions to date onboard I decided to go for the Nav3. It was the most expensive but I liked the unit. Now that I have it and I've been playing with it I find a number of disappointing limitations despite it's so call flexible setup options over the Zumo 550. The traffic receiver is not compatible, Safety Camera Subscriptions are not compatible, no car mount, no remote, unable to customise the units vehicle on maps etc. The mount for my bike is fiddley to remove the unit from the bike and the Zumo screen is way brighter than he Nav3 screen!
BMW says I can take the unit back for a refund or change it for another device - IE Zumo 550 or the BMW Zumo - which took me by suprise as I didn't expect that. The Zumo is appealing cause it will do the extras that the Nav 3 won't but now that I have the Nav3 should I persevere or return it? What else do I need to know??
You have only had it two minutes. 
The Nav III is fine for most applications.
It will work with the traffic receiver (well it did on mine)
The device should be easy enough to remove from (and put back into) its cradle. Attaching the power lead can be a fiddle nad the traffic prompt thing, too but it's not life threatening.
The Nav III screen is - or should be - bright enough. The second generation ones have the new non-reflective screen, making them much better in very bright sun. Where do you have the device sat?
Why do you want to muck around with the position cursor?
There is a car mounting kit. It is basic but it works.
Not sure what value the remote is.
I never tried to load speed camera data bases onto my Nav III. No doubt somebody has.
Zumo 550 / BuMW 550
You already have a dedicated thread on the two devices but....
The BuMW version is simply a rebranded 550. Pay the extra if you feel like it. It has the BuMW dealers pre-loaded, if that is handy. It is black.
The 550 does not have a remote.
You can fool around with the device's cursor.
It has interchangeable bling tops.
You can charge your BlackBerry (if you have one) using the same charger, but not the other way around.
It has a car mounting kit but, as far as I know, voice will not work in the car.
It will play tunes, if you load them.
It has an off-bike battery supply, which some people find handy.
It too vibrates in its cradle. Some people have had a problem with contact pins getting worn.
Sometimes it needs re-booting. Keep the Allen key handy, you will need it to take the battery out.
The cradle is fine and it's a doddle but the security screw is a pain. Replace it.
The ram-mount it comes with is ugly IMHO. Others like it.
It comes with assorted power leads (including car) but not one designed to fit the dedicated feed.
The lead to connect it into the dedicated feed (behind the head stock / oil cooler) can be bought (expensive) or made (cheap). The BuMW variant may come with one.
It's a good bit of kit but you won't get any free bike routes, unless you go looking for them.
++++++
Read the assorted threads on this site and others, like the Zumo forum, for what they are worth.
Change it if you feel like it but please do not worry and wonder so much.
It is just a box of electrical widgits, not open heart surgery, after all. You are - again - drowning in questions, half of which people cannot (or won't) answer for you. You heard that the Zumo mount was crap but then didn't go and look at one. What makes you think anything you read here will be any more reliable? Make your own mind up.
